Over the last few years, the Internet has been transformed from a simple data transmission medium to a virtual world. Goods can be bought and sold in a variety of ways through retail sites, auction sites, bulletin boards, etc. Many people use the Internet to learn, research, and explore. More recently, devices such as robots have been controlled remotely over the Internet.
The Internet is a packet-switched network. When a user sends information over the Internet to another computer, the data is broken into small packets. The Internet does not guarantee a specific delivery time or a particular delivery route for the packets. Therefore, the delays caused by communication links over the Internet are random with no simple statistical model.
Routers direct the packets across the Internet individually. When the packets arrive at the destination computer, the packets are recombined into their original form. Two different protocols handle the work of breaking the data into packets, routing the packets across the Internet and recombining the packets on the other end. Internet protocol (IP) routes the data and transmission control protocol (TCP) breaks the data into packets and recombines the packets at the destination computer.
Each packet of data is assigned a header that contains information such as the order of reassembling the packets. As TCP creates each packet, it also calculates and adds a checksum to the packet. The checksum is a number that TCP uses to determine whether an error occurred during transmission. The checksum is based on the precise amount of data in the packet. Each packet is enclosed in a separate IP envelope that contains address information for instructing the routers.
As the packets are sent across the Internet, the routers examine the IP envelopes and look at their addresses. The routers determine the most efficient path for sending each packet. After traveling through a series of routers, the packets arrive at the destination computer. Because the traffic load on the Internet varies constantly, the individual packets may be sent along different routes and may arrive at the destination computer out of order.
As the packets arrive at the destination computer, TCP calculates the checksum for each packet. If the calculated checksum compares favorably with the checksum contained in the packet, the TCP packet does not contain errors. If the checksum does not match, TCP knows that the data in a packet has been corrupted during transmission. TCP discards the packet, contacts the sender, and requests that the corrupted packet be retransmitted. When the destination computer receives all of the non-corrupt packets, TCP assembles the packets into their original form.
Real-time control systems over the Internet must account for the delays, packet loss and other transmission problems. The transmission problems are particularly important in closed-loop super-media systems. Super-media systems include feedback of audio, video, text, and sensory information such as temperature and/or haptic. These systems must also contend with lost packets and random disconnection. Each of these transmission problems may cause instability in the control system, especially for closed-loop systems.
There are several conventional systems that employ robots that are controlled remotely through the Internet. In one system, a semi-autonomous telerobot responds to trajectory commands that are provided by a remotely located operator through the Internet. The operator intervenes only when unexpected circumstances arise. This system and others like it typically provide visual information but no other form of sensory feedback.
A common form of sensory feedback is force feedback that may correspond to many different environmental parameters. Force/torque sensors are one of the most common ways of generating force feedback. One example of force feedback is provided by computer games that use the Microsoft Sidewinder® Force Feedback Pro joystick. Force feedback can correspond to actual or virtual forces. For example, force feedback corresponding to an actual force may allow the operator to a feel a force that corresponds to a physical force that is exerted on the controlled device or robot. Force feedback corresponding to a virtual force may represent the distance between the controlled device and an object. Force feedback couples the operator with the environment by allowing the operator to feel the environment and the status of the controlled device. In other words, force feedback provides important haptic information that can significantly improve the efficiency of teleoperation.
There have been many attempts to control devices over the Internet with force feedback. Each of the conventional approaches has made one of the following assumptions: the transmission delay is fixed and not random; the transmission delay is the same in both directions; or, the transmission delay has an upper bound or limit. None of these assumptions are true when dealing with real-time closed-loop control systems over the Internet that employ force feedback or other super-media feedback.